1. Field of the Invention
The invention concerns a method to determine the value of the tube voltage of an x-ray tube of an x-ray device in order to acquire x-ray projections in a multiphase examination of defined tissue a patient. The invention also concerns an x-ray device (in particular a computed tomography (CT) apparatus) which is designed to execute such a method. The invention also concerns a non-transitory, computer-readable data storage medium and coded with programming instructions that, when the data storage medium is loaded into a computerized control unit of an x-ray device, caused the x-ray device to be operated in accordance with a method of the above-described type.
2. Description of the Prior Art
In medical technology, when imaging with x-ray radiation, for example in computed tomography, it is always sought to apply an optimally low dose of x-ray radiation to a patient to generate one or more images of a tissue of the patient. However, a defined image quality must also be achieved in order to be able to answer the clinical question that is the reason for generating the image or images, and this requires a minimum dose. Relevant measures of image quality are the image noise or the image contrast, for example. The image contrast of defined elements and materials (such as the iodine used as a contrast agent in computed tomography) has a relatively strong dependency on the spectrum of the x-ray radiation that is used. Given the use of relatively low tube voltages, the spectrum of the x-ray radiation is such that the image contrast of iodine increases. Therefore, given the use of iodine, the dose of x-ray radiation that is applied to a patient to achieve an optimally high image quality also depends on the spectrum of the x-ray radiation.
In CT angiograms to show blood-carrying vessels, in which the visibility of iodine is of primary importance, the dose of x-ray radiation applied to a patient is therefore reduced by the use of relatively low tube voltages (see M. J. Siegel et al., “Radiation Dose and Image Quality in Pediatric CT: Effect of Technical Factors and Phantom Size and Shape”, Radiology 2004; 233: Page 515 to 522).
In “Automatic selection of tube potential for radiation dose reduction in CT: A general strategy”, L. Yu et al. propose a method to select a tube voltage suitable for a specific examination with regard to a reduction of the dose of x-ray radiation that is to be applied to a patient. In this method, an iodine contrast-to-noise ratio is used as an image quality index in connection with a noise constraint parameter α in order to be able to characterize and account for the different requirements for the image quality for different examinations. Different noise constraint parameters α are available for adaptation for different examinations. Using the noise constraint parameter, it is sought to make the absolute image noise not exceed a certain value. For a CT angiogram in which the iodine contrast-to-noise ratio is of primary relevance, the noise constraint parameter α is selected between 1.5 and 2.0. The noise constraint parameter α is selected between 1.1 and 1.25 for breast, torso or pelvic examinations with contrast agent, and α is selected equal to one for breast, torso or pelvic examinations without contrast agent. The tube voltage with which the lowest dose of x-ray radiation is applied to the patient for a specific examination is determined based on a “relative dose factor” (RDF) into which the contrast-to-noise ratio of iodine and the noise constraint parameter α enter.